You would have to run it through an encoder, and set the output format to NTSC. If you use Nero Ultra Edition, I believe it comes with Nero Recode, which you could use.
Oh, if you mean the actual DVD itself, you would have to use a program to RIP the DVD to a video file on your computer, then encode it with an encoding program, and then burn a new DVD with the file you created by encoding it.
VideoHelp.com - Forum, Guides, Tools and hardware lists has many documents on dealing with video conversion including converting a PAL DVD to NTSC. I'v enever done it personally but some of their other docs have helped me solve similar problems.
One program I have found, though I have not tested it, is All Video Converter. It supposable can convert different types of video formats, including PAL to NTSC.